The economical machining of internal threads in a small diameter bore of a workpiece, can be accomplished by mounting a triangular shape insert in a corresponding pocket of a bar or the like. Then the bar is fed into the bore of the workpiece. When a cutting edge of the insert becomes worn, a fastener is loosened, the insert is rotated 120xc2x0, and the fastener is retightened with a fresh cutting edge positioned to cut a thread. The insert has a horizontal bottom surface and a pair of vertical locating surfaces that are tightly clamped to corresponding positioning surfaces in the pocket of the tool holder bar. The clamped surfaces are intended to provide stability for the insert to resist deflection and avoid vibration under the very high forces encountered during thread cutting. Although the bottom location surface comprises a large area, the side locating surfaces of prior art inserts are less than adequate due to the thread form being ground into the sides of a triangular shape insert blank. The insufficient locating surface remaining, is subjected to very high forces per unit area where it presses against corresponding positioning surfaces of the tool holder pocket. If the area of the locating surface could be increased, for a triangular insert of predetermined size (i.e. which can fit into a hole of predetermined size) then threads could be cut with a much smaller possibility of damage due to excess forces between the insert locating surfaces and the tool holder positioning surfaces.
For ease in machining, the locating surfaces on the insert and the corresponding positioning surfaces in the pocket of the tool holder are machined flat. It can be appreciated that the insert should be constructed so its locating surfaces can be easily ground precisely flat, and that the flatness and the angles between the different locating surfaces can be easily measured.
In accordance with one embodiment of the present invention, a threading insert of generally triangular shape and with three cutting arms is provided, wherein a mount portion of the insert, from which the arms project, forms long locating surfaces, where the locating surfaces can be easily precisely machined and measured. The mount portion has triangle sides lying on the sides of an imaginary triangle that has an axis, with each cutting arm extending generally radially outward from the mount portion and having an outer edge adjacent to a corner of the imaginary triangle. The outer edge of each arm has a location that projects beyond the side of the imaginary triangle. As a result, the insert of the present invention can not be readily ground from a blank of triangular shape, as in the prior art. Instead the present insert is produced from a special blank of a special preformed shape.
The locating sides at the three triangle sides of the mount each has a height less than the total height of the mount, with the top and bottom of each locating side connected by a concave border to the upper and lower faces of the mount. Also, horizontally spaced opposite ends of each locating surface are each connected by a concave border to an arm. This results in each locating face being distinctly separated from surrounding portions of the insert. The upper face of the mount portion is preferably formed in a hexagon shape with substantially equal hexagon sides.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.